1. Field of the Invention
The present invention relates to anti-islanding inverters, power converters and generators connected to electric distribution utility grids.
2. Background Art
The distribution of electric power from utility companies to households and businesses utilizes a network of utility lines connected to each residence and business. This network or grid is interconnected with various generating stations and substations that supply power to the various loads and that monitor the lines for problems.
An electric utility grid generally can also consist of many independent energy sources energizing the grid and providing power to the loads on the grid. This distributed power generation is becoming more common throughout the world as alternative energy sources are being used for the generation of electric power. In the United States, the deregulation of electric companies has spurred the development of independent energy sources co-existing with the electric utility. Rather than have completely independent energy sources for a particular load, these alternative energy sources can tie into the grid and are used to supplement the capacity of the electric utility.
The number and types of independent energy sources is growing rapidly, and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, super-conducting, flywheel, and capacitor types, and mechanical means including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. In many cases these energy sources can sell the utility company excess power from their source that is utilized on their grid.
Each of these independent energy sources needs some type of power converter that feeds energy to the grid to power the various loads and also needs to provide protection when the grid becomes unstable. The utility company is still the main power source and in many cases controls the independent source to some degree. Safety concerns arise when the utility source is disconnected from the independent sources, leaving the independent source directly tied to a load or disjointed grid branch. There are various passive and active means that can be used to disconnect the independent energy sources from the grid once the utility connection is lost.
Utility companies are concerned that power converters used for distributed electric power generation (e.g. converting power from photovoltaics, micro-turbines, or fuel cells at customer sites) may continue to generate and feed the grid, or part of a grid, even if the utility connection to the grid is disabled. This condition is known as islanding or "run-on".
Islanding is more generally defined as the continued operation of a grid-coupled power converter, generator or independent power source in general, in cases where the utility grid has been switched off, cut-off, or the distribution lines have been damaged, so that no electric energy is delivered from the utility side. The electric utility industry has long recognized the need for eliminating islanding conditions on electric utility grids.
There are many hazards associated with islanding to human life, property, and equipment. These concerns are based on a fear that power quality (i.e. voltage, frequency or harmonic content) might go outside acceptable bounds, or that a line service person working on a line thought to be dead could be harmed from a backfeed from the distributed generator, or that their lack of control of an island may make reconnecting the system and resolving the problem difficult or impossible. A section of the grid that is disconnected from the utility may be undergoing maintenance or repair. If there is an energy source connected to the grid, utility personnel can be injured directly or indirectly from an electrical shock.
There is also a hazard to load equipment that could be damaged by unstable grid voltage or frequency once the utility is disconnected. The distributed generating equipment itself can be damaged due to fluctuations in the power which may be also a safety hazard to the property and persons nearby such equipment.
Power converters such as inverters are necessary in modern power systems as new energy generating devices such as photovoltaics, micro-turbines, fuel cells, superconducting storage, etc., all generate DC electricity which needs to be converted to AC for feeding into the power grid. DC-AC inverters generally behave as a current source that injects a controlled AC sinewave current into the utility line. The controlled AC current is generated in sync with the observed utility zero crossings, and may be exactly in phase, generating at unity power factor where upon real power only is exported. It is also possible to generate a variable amount out of phase; at other than unit power factor where upon real and reactive power is exported to the grid. An effective change in reactive power output can be made by either phase shifting the output current waveform with respect to voltage, or by creating an assymetric distortion to the output current waveform.
Inverters that perform this DC-AC conversion function are known as "Utility-Interactive Inverters" and are the subject of several U.S. and international codes and standards, e.g., the National Electrical Code. Article 690--Photovoltaic Systems, UL 1741, Standard for Photovoltaic Inverters, IEEE 929--Recommended Practice for Utility Interface of Photovoltaic (PV) Systems.
This invention applies not only to DC-AC inverters, but also to many other methods of conversion to AC electric power where interconnection with a power grid is involved. Examples are static inverters, and rotary converters (DC-AC motor-generator sets) which convert DC electricity to AC electricity; cycloconverters and AC to AC motor generator sets, which convert AC electricity to AC electricity; and mechanical generators which convert mechanical energy to AC electrical energy. The general term "power converter" or "converter" is used herein to indicate such devices.
There are many schemes that have been proposed to eliminate the islanding problem. The simplest use "passive methods" such as under and overvoltage, and under and over frequency shutdown trips of the converter equipment if the voltage or frequency exceed certain predefined limits. For example, proposed limits under IEEE P929 are +/-0.5 Hz from nominal for frequency and approximately +10/-14% from nominal for voltage.
More complex schemes use "unstable frequency" or "active frequency drift", where the converter frequency control circuit is made unstable so that an island condition will tend to drift up or down in frequency, ultimately creating an under or over frequency shutdown.
Other schemes involve changing the real or reactive power output of the converter to create a change in converter terminal voltage, and in most cases, an under or over frequency voltage shutdown.
Studies have found that although these methods work in most circumstance, these currently-used schemes still have a non-detect zone, where the power source will continue to supply the grid even after the utility source has been disconnected.
For example, one implementation discloses using a slide-mode frequency shift method, whereby the phase of the converter's output current is made to be a function of frequency. The converter's voltage-current phase relationship increases or decreases faster than the load's, making the 60 Hz equilibrium point an unstable point. The slide-mode frequency shift method has a non-detection zone when feeding high-Q utility circuits such as may occur on lightly loaded feeders with power factor correction capacitors as a resonant RLC parallel circuit formed by the load, transformer magnetizing inductances and the power factor correcting capacitors respectively. Q is the "quality factor" of the resonant circuit and is defined as: EQU 2.pi.FL/R
where F is the utility and resonant frequency, L is the circuit inductance and R is the load resistance. Islanding has been observed in lab experiments with Q values as low as 2.0. Such values are possible in real world utility distribution circuits.
Another implementation discloses changing reactive power based on the rate of change of frequency. This technique differs from these herein disclosed as the present invention uses a measure of change in frequency, rather than rate of change of frequency which is the first derivative of same.
Another problem that has not been fully addressed is that of multiple converters on an electric power grid. For example, if the unstable frequency method is used, and one manufacturer chooses a tendency to drift up in frequency, and another, a tendency to drift down in frequency, their efforts may balance and the island will remain, undefeated.
In order to reduce the aforementioned problems, attempts have been made to produce reliable anti-islanding converters. Generally, these have been effective under most conditions. However, theoretical analysis has shown that all have a considerable non-detection zone that, to this point, has yet to be satisfactorily addressed. Utility companies are requiring certain shutdown in case of loss of grid power, so any non detection zone is unacceptable. A non-detection zone is an area of operation with specific loads that is capable of sustained islanding. This is typically a range around the resonance and real and reactive power match points of an RLC load circuit.
One such invention is described in U.S. Pat. No. 5,493,485 discloses an inverter device that incorporates a detection circuit and a correction circuit that generates a corrective signal to disconnect the source from the grid upon detecting the utility source has been disconnected. The correction circuit relies on an initial reference signal.
U.S. Pat. No. 5,162,964 uses frequency thresholds for under frequency and over frequency conditions. The pre-determined or set thresholds will disconnect the source from the grid only when the threshold is crossed.
These two patents use simple over and under voltage and over and under frequency limits which have been shown to be inadequate to fully protect against islanding.
The inverter device of U.S. Pat. No. 4,878,208 initially measures the AC output signal and compares present phase response to the initial measurement. If the present phase is outside a pre-determined range, the unit disconnects the source from the grid. This is a widely used and effective technique that is integral in most inverter designs as if phase-lock is lost, abnormally large currents may be drawn. Such a phase shift will occur if a non-unity power factor load and power converter are suddenly disconnected from the grid.
European Patent EP 810713 uses a distortion source to cause a variation in the output frequency upon detecting an islanding condition. The distortion encompasses forms of feedback loops and pulse width modulation variation. Applying distortion to the converter output waveform will not necessarily result in reliable island detection as the voltage response of the islanded load is dependent upon its impedance. A very high impedance at the distortion frequency, or the additional distortion caused by a non-linear load could defeat this scheme.
There are several inverter products available that have reduced the islanding problem, including the Ascension Technology SunSine300 and the Advanced Energy Systems MI-250. The Advanced Energy Systems (AFS) product uses an active unstable frequency shifting technique that increases frequency once the utility is disconnected, eventually causing an over-frequency trip in most cases. The direction of frequency drift is settable in software. However, studies of the current product offering indicated that none of the units were adequate to prevent islanding within tolerable limits.
For example, an AES inverter when connected to a worst-case load (a test required by one major US utility) consisting of an induction motor with flywheel loading, plus compensating capacitors to bring the power factor to unity, and some parallel load resistance was found to island indefinitely.
What is needed is an anti-islanding scheme that can effectively and reliably eliminate islanding conditions in all likely distributed generation situations. This scheme should be flexible in usage, widely applicable, cost-effective to manufacture and capable of integration into all types of existing and future converter systems.